Characterization of Complex Soil Stratigraphies by VisCPT and Adaptive Remeshing

最先进的岩土工程和地质环境场地表征现在普遍采用直接推动技术。 静力锥贯入试验（CPT）及其变体--压电锥（uCPT）和地震锥（SCPT）--是直推技术的领先者，可提供最全面的场地地层评估和工程（机械和地质水文）土壤特性测定。 然而，如果土壤沉积物是复杂的，CPT往往无法提供准确的地层信息，并产生不可靠的属性。 这些复杂性可能包括不寻常的现场应力历史（地貌）和地层薄层。 目前的研究工作利用两个最近开发的技术来创建程序，全面评估网站地层和确定土壤特性，即使是薄土层，如砂缝和粘土透镜体。第一个工具是有限元建模中的“自适应网格重划分”。较硬的异物（如钢锥比重计）对土壤的大量渗透会在较软的材料中产生较大的位移和应变。 因此，有限元网格变得非常压缩和扭曲，导致数值不稳定。 然而，通过自适应地重新划分土壤网格，可以模拟米量级的渗透。 因此，本研究将利用该工具来研究土壤对CPT的反应。将模拟穿过不同土壤之间的界面以及穿过薄地层的渗透，以了解在这种条件下的典型CPT特征。 将探讨正常固结和超固结应力状态。将自适应重新网格划分结果外推到现场条件，特别是确定薄地层的工程性质，需要了解这些地层的实际厚度和位置。 为此，将使用最近开发的“视锥光度计”（VisCPT）。 以前的研究已经导致这种增强传统的CPT，它允许连续的图像记录的土柱的发展。 VisCPT甚至可以轻松检测到薄至0.5 cm的层，从而提供传统CPT所缺失的信息。 一旦确定了土层厚度，就可以通过有限元研究的结果和自适应网格重新划分来预测土壤特性。这项研究的更广泛影响包括广泛的跨学科研究，立即将技术转移到行业，以及重新设计本科实验室经验，以包括VisCPT技术。 改善场地特征将导致更安全和更具成本效益的土木工程建设。 有了更精确的地下信息，建筑行业将能够降低目前需要防止意外情况的典型高安全系数。

State-of-the-art geotechnical and geoenvironmental site characterization now ubiquitously employs direct-push technologies. The cone penetration test (CPT) and its variants, the piezo-cone (uCPT) and seismic-cone (SCPT), lead the direct-push technologies in providing both the most comprehensive assessment of site stratigraphy and determination of the engineering (mechanical and geohydrologic) soil properties. However, if the soil deposits are complex, the CPT often fails in providing accurate stratigraphic information and yields unreliable properties. These complexities may include unusual site stress history (geomorphology) and thin layering of strata. The present research effort utilizes two recently developed technologies to create procedures for comprehensively assessing site stratigraphy and for determining the soil properties of even thin soil layers such as sand seams and clay lenses. The first tool is "adaptive remeshing" in finite element modeling. Large penetration of soil by stiffer foreign objects such as a steel cone penetrometer creates large displacements and strains in the softer material. Therefore, a finite element mesh becomes very compressed and distorted leading to numerical instability. However, by adaptively remeshing the soil, penetrations on the order of meters can be simulated. The present research will therefore utilize this tool to study the response of soils to the CPT. Penetration across interfaces between dissimilar soils as well as through thin strata will be simulated to gain an understanding of the typical CPT signature in such conditions. Both normally consolidated and overconsolidated stress states will be explored.Extrapolation of the adaptive remeshing results to field conditions and specifically the determination of engineering properties of thin strata will require knowledge of the actual thicknesses and locations of these layers in-situ. For this, a recently developed "vision cone penetrometer" (VisCPT) will be utilized. Previous research has resulted in development of this augmentation to the conventional CPT, which allows for continuous image recording of the soil column. Even layers as thin as 0.5 cm are easily detected by the VisCPT thereby providing the missing information from the conventional CPT. Once a layer thickness is established, the soil properties can be predicted by the results of the finite element studies with adaptive remeshing.The broader impacts of the study include extensive interdisciplinary research, immediate technology transfer to industry and redesign of undergraduate laboratory experience to include VisCPT technology. Improvements to site characterization will lead to safer and more cost effective civil engineering construction. With more precise subsurface information, the construction industry would be able to reduce the typically high factors of safety that are presently needed to protect against unexpected conditions.